1. Field of the Invention
The present invention relates to an organic light-emitting display device, and more particularly to an organic light-emitting display device including a dye-based polarizer capable of maintaining a polarization degree even at high temperatures.
2. Discussion of Related Art
An organic light-emitting display device, which is a next generation display device having self light-emitting characteristic, has better characteristics than a liquid crystal display device (LCD) in terms of viewing angle, contrast, response speed and power consumption, and can be manufactured to be thin and lightweight since a backlight is not required.
The organic light-emitting display devices can be categorized as a passive matrix type in which pixels are arranged in a matrix format at locations where scan lines and signal lines cross each other, and an active matrix type in which each pixel is controlled by a thin film transistor (TFT) that operates as a switch.
In the passive matrix-type organic light emitting display device, scan lines are sequentially driven over a period of time to drive each pixel, but in the active matrix-type organic light emitting display device, each pixel is driven by using a storage capacitor. This way, the active matrix-type organic light emitting display device is capable of obtaining the same brightness with low current so that it has advantages in terms of less power consumption, high definition and large-size over the passive matrix-type.
FIG. 1 is a schematic cross sectional view of one example of a conventional passive matrix-type organic light-emitting display device.
Anode electrodes 11 are formed on an insulating substrate 10, and a thin organic layer 20 is formed on the anode electrodes 11. The thin organic layer 20 is formed to have a structure in which a hole transport layer 21, an organic light-emitting layer 22 and an electron transport layer 23 are stacked, and can further include a hole injection layer and an electron injection layer. Also, on top of the thin organic layer 20, cathode electrodes 30 are formed to cross with the anode electrodes 11. The above described organic light emitting display device is encapsulated by a sealing substrate 40.
FIG. 2 is a schematic cross sectional view of one example of a conventional active matrix-type organic light-emitting display device.
A thin transistor T including a semiconductor layer 51, a gate electrode 52, and source and drain electrodes 53, 54, is formed on an insulating substrate 50. The source electrode 53 of the thin transistor T is connected to a storage capacitor CST having a structure in which an electrode 55, a dielectric 56 and an electrode 57 are stacked. The drain electrode 54 is connected to a light-emitting element E having a structure in which an anode electrode 58, an organic layer 59 and a cathode electrode 60 are stacked. The above described organic light emitting display device is encapsulated by a sealing substrate 70.
In the organic light-emitting display device constituted as above, when a predetermined voltage is applied to the anode electrode and the cathode electrode, holes injected through the anode electrode and electrons injected through the cathode electrode are recombined at the light-emitting layer, thereby emitting light using a difference in energy generated during the process.
An organic light-emitting display device can be categorized as a bottom-emitting type wherein the emitted light is propagated towards the lower surface, or a top-emitting type wherein the emitted light is propagated towards the upper surface.
In the bottom-emitting-type device, since the emitted light is emitted towards a substrate on which the thin film transistor is formed, a wiring portion including the thin film transistor is excluded from a display area, while in the top-emitting-type device, the emitted light can be emitted towards the upper surface over the thin film transistor, thereby obtaining a wider display area.
A contrast ratio in the organic light-emitting display device, which is a brightness ratio at the time of on/off, represents a degree to which an image can be clearly recognized. The brightness at the time of off is determined by reflectivity to an incident light from an external light source. Therefore, in order to raise the contrast ratio, reflection of light from the external light source should be reduced.
In order to reduce the reflection of light from the external light source, a use of a λ/4 phase difference plate or a polarizing plate on the plate surface in the path through which the emitted light is propagated (See Korean Patent Application No. 2000-60524 (Oct. 14, 2000)) or a use of a circularly polarizing plate capable of controlling the phase difference, has been proposed.
In the case of using a polarizing plate that allows a light having a specific polarization direction (for example, only a horizontal wave) to pass through, the horizontal wave passing through the polarizing plate, is extinguished because it does not pass through the polarizing plate again, since the horizontal wave is reflected by an internal metal electrode and its phase is then changed using the λ/4 phase difference plate. Therefore, the phase difference is induced using the λ/4 phase difference plate and thus, the incident external light cannot be emitted, resulting in a reduction of reflection of the external light.
However, the organic light-emitting display device constituted as above has a disadvantage in that since even the light emitted internally from the organic light-emitting display device is interrupted by the polarizing plate to approximately 50%, its brightness is reduced by at least one-half. Also, the conventional polarizing plate is constituted of an iodine-based polarizing plate and thus, has poor durability against heat and humidity. Hence, it is difficult to apply such polarizing plate in display devices (e.g., television, etc.) to be used in environments that are exposed to heat (e.g., solar radiation) or humidity.